https://sourceforge.net/p/tcl/mailman/message/37738603/ (Tcl/Tk 8.6.13 Release Announcement) with the tl;dr of: This is a patch release, so it primarily includes bug fixes and corrections to erratic behavior.

Also, new reddit can die in a raging fire: https://old.reddit.com/r/Tcl/comments/z2459z/tcltk_8613_rele... but I'll save you the click, it's just someone who posted the mailing list message on reddit